To: | david jordan <wa3gin@erols.com>, keith@dutson.net |
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Subject: | Re: [TowerTalk] spider balls.. they work |
From: | Jim Lux <jimlux@earthlink.net> |
Date: | Fri, 30 Jul 2004 09:59:02 -0700 |
List-post: | <mailto:towertalk@contesting.com> |
At 10:13 AM 7/30/2004 -0400, david jordan wrote:
Boys, NASA does NOT use these devices any more. There was a test done quite a few years ago (Apollo era) and after several strikes on the dissipator itself, extensive review, etc. they went to a more conventional "Franklin type" (i.e. rod, good grounding, etc.) lightning protection scheme. They actually have video of lightning striking the dissipators. In the FAA installations often cited by the mfrs of the dissipators the same thing is true. There are also some odd things like employees of the mfr later going to work for the government and recommending the installation of the products of their former company. I've started to collect some information on this. http://home.earthlink.net/~w6rmk/lightning.htm has links It seems that the manufacturers of the dissipators are legally very aggressive, having threatened lawsuits against IEEE and NFPA, among others. This prompted a more thorough than usual review of the IEEE paper (by Mousa... google for Mousa and Lightning... it will turn up). Interestingly, Mousa does say that the dissipators can have a beneficial effect on a certain class of discharges, but actually makes another kind worse, and, overall, that you're better off with a well designed conventional system. Also, for what it's worth, the special class of discharges apparently cannot occur when the thing being discharged is less than 300 meters tall. The mfr of the dissipator tried to get a NFPA standard for the dissipators (as opposed to the existing NPFA 780 lightning rod standards), and when a huge number (probably every lightning expert in the world) of very knowledgable people, backed up by extensive research, said that the proposed standard was, in short, "bunk". Here are several web sites that have good descriptions of how the devices are used and the expectations and performance of the devices. Nott is combining the usefulness of static dissipators for such things as P-static and windblown dust/rain charging on insulated antennas, with their usefulness for preventing lightning strikes. His statement: "Unfortunately, a large amount of misinformation has accumulated over the years, much of it now accepted as fact by virtue of tradition." is more true than I think he appreciated when he wrote it back in 1986. "http://www.lightningmaster.com/Broadcast-2.htm", Is quite vague, and has several misinterpretations of lightning theory. I note also an interesting statement: " If the dissipator acts as a lightning rod and takes a hit (all static dissipation systems should for designed lo withstand this possibility of lightning will follow the path of least impedance lo ground. " Doesn't fill you with confidence that that the things actually work, does it. "http://www.esda.org/esdbasics3.htm", This article talks about ESD protection, where the charging source is limited both in physical extent, size, and duration. A thunderstorm, extending over thousands of meters, with couloumbs of stored charge, and continuous charging currents of amps, is hardly in the same category. and here is one written by a fellow ham operator "http://www.repeater-builder.com/rbtip/static.html". This is describing a dissipator for P-static, which is a real problem, and for which fuzzy pointy things work great. Too bad that Kevin added the totally erroneous statement: "Although not marketed as a lightning deterrent, it is interesting to note that after this device has been installed, lightning will likely NOT strike this antenna because of the lack of ability of the antenna to build a counter charge where discharge from a cloud would be attracted." _______________________________________________Good Luck, dave wa3gin See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA. _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk |
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